(1) Field of the Invention
The present invention relates to a process for improving the resistance to corrosion of stainless steels. The process may be widely employed for preparing metallic substrates for use in the making of tableware.
(2) Description of the Prior Art
The stainless steels designated "SUS 304 steel" in JIS G4303 and having the composition as shown in Table 1 have been known as corrosion-resistant metallic materials, widely used as substrates for preparing TiN-coated stainless steels by ion plating process.
TABLE 1 ______________________________________ (% by Wt.) ______________________________________ C Si Mn P S Ni Cr &lt;0.08 &lt;1.0 &lt;2.0 &lt;0.04 &lt;0.03 8.0-10.5 18.0-20.0 ______________________________________
As is well known to those skilled in the art, various metals or metal compounds can be coated by ion plating the desired thickness onto a metallic substrate.
By ion plating, the metallic substrate can be isolated and protected from a severely corrosive environment, thus retaining its useful properties for a long period of time. Therefore, ion plating is a popular method used in developing various corrosion-resistant materials for preparing metallic articles.
In TiN-coating by conventional ion plating processes, when the amount of nonmetallic impurities such as sulfide-base impurities is too large in the steel, the environment-isolating effect of the coating cannot be exerted sufficiently under wet conditions due to the presence of defects caused by the impurities.
Therefore, conventional ion plating processes do not provide improved corrosion resistance to the metallic substrate for preparing metallic articles which can be used under wet circumstances.